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Planetary News: Cassini-Huygens (2005)New RADAR Images of Titan:The Features Are Getting More Familiar, But Aren't Getting Less MysteriousBy Emily LakdawallaNovember 2, 2005 In a much-anticipated encounter, Cassini zoomed within 1,353 kilometers (841 miles) of Titan last Friday and achieved the first RADAR imaging of the Huygens landing site. Have all of the mysteries of the Huygens landing site been answered? In a word, no. "It's not easy!" remarked Rosaly Lopes, a member of the Cassini RADAR team. "We didn't really expect it to be." The following images show all of the available views of the Huygens landing site from different Cassini and Huygens instruments. First, for comparison, here is the best available view that was put together from dozens of individual frames from Huygens' Descent Imager / Spectral Radiometer (DISR):
The fortunate fact that Huygens appeared to descend across the border between bright regions and dark regions is helping match the DISR panorama to images from the Cassini orbiter. But that is pretty much where the good luck ended for those interested in locating the Huygens landing site on the orbiter's images. Titan’s cloud deck proved to be much lower than the Huygens team expected. As a result, their views from higher altitudes -- which would have spanned a larger area than the lower-altitude views -- could not see the surface. Furthermore, the thick haze, turbulent atmosphere, and the unexpected backwards rotation of Huygens during its descent stymied its Sun sensor, so the compass orientation of the Huygens images is only approximately known, making the match even more difficult.
Huygens’ final mosaics contain beautiful detail, but the images are
in fact so detailed and of such a small area that they can’t be matched
precisely to the much fuzzier orbiter views. The map at right is the
best published view from the Cassini
Imaging Science Subsystem (ISS). The
yellow dot on the map represents the guess at where Huygens would land
based on predictions of Huygens' descent path prior to the Huygens mission
on January 14.
This map is the best published view available from the Cassini Visual
and Infrared Mapping Spectrometer (VIMS). The red circle on the map represents
the guess at the landing site location after the Huygens landing based
on comparison of the VIMS and DISR images.
Finally, this is the Synthetic Aperture RADAR (SAR) image of the Huygens landing site. The yellow outline and magenta cross mark the best guess of the RADAR team at the location of the Huygens DISR images. But Lopes was cautious about the lines drawn on the map, saying "the location of the landing site is more uncertain than the map implies, so this is preliminary. It may be revised in the future." All of the maps and images above are shown at different scales, making comparisons
difficult. Here are comparison images showing the available image data
at the same scale and covering approximately the same areas. Each image
represents an area 500 kilometers (310 miles) square.
A first impression, comparing the SAR image to the VIMS and ISS images, shows that the patterns of brightness and darkness, also known as "albedo," in the VIMS and ISS images do not match the broad variations in the SAR images. "What's light and dark in RADAR is not necessarily what's light and dark in visible wavelengths," Lopes pointed out. "This makes it kind of difficult to pinpoint the exact location of Huygens."
However, closer investigation of the images reveals that there are a few clear correlations. Large areas of the SAR image are covered with "cat scratches," east-west aligned sets of parallel, dark lines. Cat scratches in SAR images broadly correlate with low albedo in the ISS and VIMS images. In fact, everywhere the science team looked in the new SAR images, "we see lots of cat scratches, really lots of them," Lopes said. "They are starting to look pretty common on Titan, but mostly at lower latitudes. We still think that they are aeolian but there is still debate about that." The team is yet lacking some data that would be useful in interpreting what the cat scratches are. Cassini captures the data for SAR imagery by broadcasting a radio signal at Titan and listening for its echo; the effect is similar to seeing in the dark using a flashlight. Almost all of the SAR swaths that Cassini has acquired to date were captured when Cassini was flying in an east-west direction across Titan. As a result, nearly all of the SAR swaths are illuminated from the north or south. Lopes is looking forward to a future SAR swath captured from a different illumination direction. "We are wondering whether looking at the cat scratches from a different direction would help us constrain [their origin] better. Because at the moment we are only seeing them perpendicular to the SAR. They are east-west and we are looking north-south. Maybe we wouldn't see them if we were looking in another direction." The most exciting features Lopes saw in the new SAR swath were "long ridges that we think are tectonic. We have not seen those before. We can't tell how high they are, but we think they are ridges of hills. In the SAR they look bright." The ridges or hills seem to crop up out of smoother regions filled with cat scratches. Lopes noted that the cat scratches appear to be diverted by these hills and flow around them, supporting both the notion that the hills are in fact topographic features and the theory that the cat scratches are aeolian and aligned with the motion of surface winds. Unfortunately, it is not yet possible to be certain of the topography across areas seen in the SAR images, because Cassini cannot acquire SAR images and altimetric measurements simultaneously. "As the mission goes on we'll begin to get altimetry over the SAR, and that's when it will get really interesting," Lopes said. But what the RADAR team is really waiting for is the opportunity to see two regions on Titan at different RADAR "look angles." This will only happen when two SAR swaths overlap. The first opportunity to get overlapping SAR coverage does not occur for another year, during the Titan-20 ("T20") flyby on October 26, 2006. Cassini mission planners have not yet decided whether to take advantage of this opportunity.
"We planned the T20 flyby two ways," Lopes said. "It's a very good flyby for a number of instruments, but of course we can't acquire optical remote sensing and RADAR data at the same time, so we will have to chose one or the other." The decision will be made in January of 2006. "We are hoping that the project science group will decide in our favor. If we get it, we will have a new SAR swath that will cross over that big impact crater we saw in T3," the Titan flyby that occurred on February 15, 2005. Each Titan flyby delivers loads of new data, but rather than answering questions the data always seems to whet our appetites even more. Fortunately, this was only the ninth of 45 planned flybys. The next Titan flyby occurs on December 26. |
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